The invention relates to a process for the preparation of urethanes and/or ureas by oxidative carbonylation of amino compounds in the presence of carbon monoxide and oxygen as oxidizing agents and, particularly in the case of the urethanes, in the presence of an organic compound containing one or more hydroxyl groups.
The preparation of isocyanates, urethanes (which are also referred to as carbamates) and also ureas on an industrial scale currently includes the reaction of the corresponding primary amine with phosgene. Large amounts of hydrogen chloride are formed as a by-product of this reaction and must be disposed of or used further in coupled processes in a relatively expensive manner. There is therefore a great interest, on the part of industry, in developing non-coupled synthesis processes which can be carried out without the use of phosgene and which avoid the by-product hydrogen chloride.
Ottmann et al. (U.S. Pat. No. 3,481,967) describes a process for the preparation of aromatic isocyanates in which aromatic nitro compounds and carbon monoxide can be prepared in the presence of transition metal catalysts. These transition metal catalysts include cobalt iodide and titanium tetrachloride. However, industrially useful processes must have a high conversion and a selectivity of over 90%, where possible, in order to be used successfully. To date, only reductive carbonylation processes have been able to meet these requirements. Such reductive carbonylation processes start with aromatic nitro compounds and use expensive noble metal catalysts (B. K. Nefedov, V. I. Manov-Yuvenskii, S. S. Novikov, Doklady (Chem. Proc. Acad. Sci. USSR), 234, 347 (1977)). In most cases, it is difficult to recover the noble metal catalysts. Therefore, these processes are not acceptable from an economic point of view. One exception is patent WO 2006 131 381 A1, which teaches oxidation of aromatic amines with corresponding aromatic nitro compounds as the oxidizing agent, in the presence of alcohols and acidic promoters, in order to obtain urethanes. Unlike oxygen, however, nitro compounds are comparatively expensive oxidizing agents which must be prepared in a separate process. Both constitute disadvantages.
Industrial and scientific research has therefore concentrated predominantly on the development of processes in which aromatic nitro compounds are converted into N-arylurethanes in the presence of carbon monoxide and an organic, hydroxyl-group-containing compound by oxidative carbonylation in the presence of oxygen. The N-arylurethanes can be converted into the corresponding N-aryl isocyanates in a further process step. An example of this type of process is disclosed in U.S. Pat. No. 4,266,077, in which aromatic amines are reacted in the presence of Co2(CO)8 as catalyst, an organic hydroxyl-group-containing compound and an unsaturated organic component. The latter is necessary to ensure high selectivities. In addition, it is expressly stated that no oxidizing agent, such as oxygen, is used.
Benedini et al. was the first to use [CoII(salen)] (salen=N,N′-disalicylidene-ethylenediamine) as catalyst in order to convert aromatic or aliphatic primary amines into the corresponding urethanes or ureas in the presence of methanol (F. Benedini, M. Nali, B. Rindone, S. Tollari, J. Mol. Catal, 1986, 34, 155-161; G. Maddinelli, M. Nali, B. Rindone, S. Tollari, J. Mol. Catal, 1987, 39, 71-77). Disadvantages of this process include the especially long reaction times of up to 48 hours and immensely high catalyst concentrations (up to 20 mol %). The selectivities are moderate to good and a product mixture of the corresponding urethanes and ureas is generally obtained.
U.S. Pat. No. 5,194,660 describes the oxidative carbonylation of aromatic amines in the presence of carbon monoxide and an organic hydroxyl-group-containing compound and also oxygen as oxidizing agent. Macrocyclic transition metal complex catalysts are used. One of the disadvantages of this process is its use of large amounts of lithium iodide as promoter. The process is not conducted halogen-free, and corrosion problems which can scarcely be overcome are further disadvantages.
Orejon et al. further developed the work of Benedini et al. using hydrolytically stable ligands based on 2,2′-bipyridine or 1,10-phenanthroline. In comparison with the salen ligand, these ligands lack the hydrolytically sensitive imine unit. However, in this work, express reference is also made to the use of alkali halide promoters (A. Orejon, A. Castellanos, P. Salagre, S. Castillón, C. Claver, J. Can. Chem., 2005, 83, 764-768).
A similar process is described in published patent application U.S. 2007/0293696 A1. In U.S. 2007/0293696, however, the use of a halogen-containing compound as solvent is an essential part of the process. A disadvantage is that the halogen-containing compounds used as solvents are considered harmful from an environmental point of view.